The Arrangement of Two N-Heterocyclic Carbene Moieties in Palladium Pincer Complexes Affects Their Catalytic Activity towards Suzuki–Miyaura Cross-Coupling Reactions in Water

Abstract

Palladium complexes with pincer ligands containing one pyridine and two N-heterocyclic carbene units with acetylprotected D-glucopyranosyl groups in C-C-N and C-N-C arrangements were synthesized. The complexes form diastereomers due to the twisted pincer ligands and chiral Dglucopyranosyl units. The diastereomers of the C-C-N complex are in equilibrium in solution, whereas only one of the diastereomers of the C-N-C complex forms kinetically. Deprotection of the acetyl groups in the ligands afforded watersoluble complexes with one of the hydroxide groups in the D-glucopyranosyl groups coordinated to the metal ion. In CD 3 OD, the deprotected complex with the C-N-C ligand gradually decomposed, whereas that with the C-C-N ligand was stable at room temperature. The complexes catalyzed Suzuki Miyaura cross-coupling reactions in water with turnover numbers of 75,000 for the C-C-N and 8,900 for C-N-C complexes. The C-C-N complex was not deactivated by the addition of metallic Hg meaning that the active species is the complex itself or its derivatives having the pincer ligand. On the other hand, the C-N-C complex exhibited no catalytic activity for the coupling reaction in the presence of metallic Hg, meaning that the active species are heterogeneous catalysts, such as Pd nanoparticles.

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…Nishioka's group presented an example 184 supported by a sugar‐functionalized C NHC C NHC N Pyr ligand (Scheme 54). [16] This complex shows superior catalytic property (versus 13 , Scheme 6, vide supra ) in Suzuki couplings in H 2 O. It was proposed that the trans effect brought by strongly electron‐donating arm NHC enhances the lability of the trans‐standing pyridine, which helps to generate the vacant coordination site and thus promotes the catalytic performance.…”

“…[15] By using this strategy, Nishioka et al synthesized the palladium complex 13 with acetyl-protected sugar moieties (Scheme 6). [16] The catalytic performance of 13 in Suzuki couplings in H 2 O was examined in this work. Hg poisoning test implicated that the decomposition of 13 to Pd nanoparticles is likely to occur making the catalytic system heterogeneous in nature.…”

Carbohydrate-Functionalized 1,2,3-Triazolylidene Complexes for Application in Base-Free Alcohol and Amine Oxidation

“…1,2,3-Triazolylidenes (trz’s) − are a subclass of N-heterocyclic carbenes (NHCs) that are conveniently accessible via functional-group-tolerant copper-catalyzed click reactions. ,− Furthermore, these ligands have pronounced mesoionic properties, ,,, which implies that the carbene has an adaptive donor strength that can stabilize low as well as high metal oxidation states. − These unique characteristics entailed the application of triazolylidenemetal complexes in a range of catalytic transformations including cross-coupling − and redox reactions such as amine and alcohol oxidation, ,− transfer hydrogenation, ,− and water oxidation. ,− Current trends in NHC chemistry are focusing on the development of functionalized ligand systems in which the ligand is noninnocent and can act, for example, as a potential proton shuttle during catalytic processes. − Carbohydrates are an attractive class of functional groups for such purposes because they provide ample opportunities to serve as hydrogen donors and acceptors to enhance the catalytic activity and selectivity. Surprisingly, however, only a few NHC metal complexes are known that are functionalized with a carbohydrate substituent, and even fewer of these have been applied in catalysis. ,− In particular, we are aware of only two reports that detail the synthesis and catalytic application of protecting-group-free carbohydrate NHC metal complexes. , Building on a previous stu...…”

“…4,53−67 In particular, we are aware of only two reports that detail the synthesis and catalytic application of protecting-group-free carbohydrate NHC metal complexes. 54,60 Building on a previous study on carbohydrate-functionalized triazolylidene complexes, which used exclusively acetylprotected sugar moieties, 55 we have now explored new glucose− and galactose−triazolylidene hybrid systems and disclosed protocols for the deprotection of carbohydrates on the metal complex. This work demonstrates that deprotection of the carbohydrate unit considerably enhances the catalytic activity and modifies the product selectivity, This modulation of the catalytic performance identifies carbohydrate−triazolylidene hybrids as attractive scaffolds for further optimization, in particular when considering the vast variability of the carbohydrate unit.…”